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I think that technically the nvidia card is a little bit faster. However, I would still go with the ati x800 simply because it takes up one slot in the powermac g5 not two like the nvidia one does. Also you might want to consider the ATI x850, it should be out by now.
 
I prefer ATI to NVidia. Although they might not be the fastest right now, they still make competive products.
 
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